Polycarbonate (PC) blends with aromatic crystalline polyesters are well known and are useful for their combination of processability, solvent resistance, appearance and impact strength. In most cases these blends also give good, uniform surface appearance. The thermal properties of these blends, especially under load, is a function of the glass transition temperature (Tg) of the polycarbonate-rich phase. To the extent that the polycarbonate is partially miscible with the polyester, the Tg of the PC rich phase of the blend will be reduced relative to pure PC Tg [ref. J. Applied Polymer Sci. 48, 2249 (1993)]. This will cause a drop in thermal properties like heat distortion temperature (HDT).
If one tries to address the drop in blend Tg by use of a higher Tg polycarbonate resin there may still be problems. Polyarylates and the related is polyester carbonate (PEC) resins are known to have some of the properties of polycarbonate with higher Tgs (see U.S. Pat. No. 4,430,484). However, when these polyarylates or polyester carbonates are mixed with standard alkyl terephthalate base polyesters, like PBT, there is little if any gain in HDT vs. the standard, lower Tg bisphenol A PC blends. The higher Tg polyester carbonates and polyarylates are more miscible with the alkyl terephthalate resins and the resultant Tg of the blend is about the same or lower than an analogous bis phenol-A PC-alkyl terephthalate blend.
So typically, polycarbonate/polyester blends may undergo a reduction in modulus at temperatures higher than about 50 to 180 degrees Centigrade. This is due to a reduction of the PC, polyestercarbonate or polyarylate Tg due to partial miscibility with the polyester. In bisphenol-A polycarbonate (BPA-PC) there is a significant drop in Tg on blending with polyesters, especially alkylene terephthalates. Polyester carbonates and polyarylates, with higher Tgs than the corresponding polycarbonates show an even greater drop in Tg when blended with polyesters. The drop in Tg affects properties like heat distortion temperature under load, which is dependent on high temperature modulus. Hence, it is desired to have improved blends of the polycarbonate, polyester carbonate or polyarylate/polyester type with a combination of properties including improved HDT as well as good appearance, impact and flow.
In polyester/polycarbonate blends adhesion of coatings is often a problem due to the solvent resistance of the polyesters. This has been addressed by addition of separate adhesion promoters such as polyalkylene glycols, see U.S. Pat. No. 5,348,999, and the selection of specific polyester end groups, see U.S. Pat. No. 5,674,928. However, limited adhesion of paints and coatings is still a problem in polycarbonate/polyester blends.
Thus, polycarbonate, polyester carbonate, or polyarylate/polyester blends with higher heat capability and improved coating adhesion, with retention of other desirable properties such as impact strength, dimensional stability, appearance and stiffness, are difficult to achieve. Hence, it is desired to have a polycarbonate & etc./polyester blend with a combination of properties including improved Tg, improved coating adhesion and higher HDT.
We have found that modification of a polyester resin with a small amount of a metal sulfonate salt gives blends with polycarbonate, polyester carbonates and polyarylates that have surprisingly improved Tg/HDT and better coating adhesion. These blends still have high impact strength, good stiffness and mechanical properties along with good appearance and processability.
We have also found that use of the metal sulfonate polyester copolymer modifies the rheology of the blends especially under low shear where the melt strength is enhanced. Enhanced melt strength is very beneficial in facilitating processing under low shear condition like blow molding and extrusion, it may also be useful for enhanced thermoformability.
Further many of the blends of this invention show enhanced retention of properties and appearance on weathering. Blends of metal sulfonate polyester copolymers with polyester carbonates and polyarylates show improved retention of gloss in simulated outdoor weathering vs. blends made with standard (non-aryl ester containing) polycarbonates.